Kuroshio Oyashio Transition Region Research Articles

Neocalanus cristatus (Copepoda) From a Deep Sediment-Trap: Abundance and Implications for Ecological and Biogeochemical Studies

We studied seasonal variations of the mesozooplankton swimmer community collected by a sediment trap moored at 873 m in the Kuroshio–Oyashio Transition region off the east coast of Japan from 5 August 2011 to 23 June 2013, with sampling bottles rotating at 26-day intervals. The total flux of mesozooplankton swimmers varied between 0 and 11.1 individuals m–2 d–1, with a mean of 3.1 individuals m–2 d–1 during the sampling period. In total, 89 taxa were found in the trap material, of which copepods comprised 87.1% of all swimmers on average. Among the Copepoda, Neocalanus cristatus was the most dominant taxon (76.2% of copepods on average during the sampling period), and all of them were stage C5 copepodite to adult. The species composition of the swimmers closely reflected the mesopelagic mesozooplankton of the Oyashio region. Because all N. cristatus observed in the trap were stage C5 to adult, its flux represents a time series of variations in life history and response to environmental changes at the depth of the sediment trap. The fluxes of Neocalanus species (N. cristatus, N. flemingeri, and N. plumchrus) reflected ontogenetic vertical migration, but may have been overestimates of active fluxes if they included dormant individuals that accidentally entered the sediment trap. The apparent active carbon flux of Neocalanus species ranged from 0 to 22.3 mg C m–2 d–1 during the sampling period, with a mean value of 4.9 mg C m–2 d–1.

Spatiotemporal spawning patterns and early growth of Japanese sardine in the western North Pacific during the recent stock increase

AbstractSpatiotemporal patterns in spawning of fish influence growth and survival by affecting the environment experienced by offspring during their early life stages. Therefore, identifying changes in spawning patterns can help researchers understand population dynamics and recruitment fluctuations. The Japanese sardine Sardinops melanostictus is a small pelagic fish that undergoes large stock fluctuations. Although spawning patterns are known to change spatially and temporally with stock abundance, little information is available on the processes underlying stock increases. This study aimed to describe changes in spawning pattern and early growth of Japanese sardine during the recent period of stock increase and to clarify the effects of different spawning periods on offspring growth. We examined trends in egg abundance in the western North Pacific in 2004–2018 and analyzed hatch dates and growth trajectories by otolith microstructure analysis of juveniles from the Kuroshio–Oyashio transition region (the species’ main nursery area). During the study period, the main spawning area shifted from the western to the eastern part of this region off Japan, and spawning in the eastern part roughly coincided with juvenile hatch dates. Hatch dates also shifted from mid‐March at the earliest to February and early March from 2013 onward. Although early‐hatched cohorts (which originate offshore of eastern Japan) experienced slower initial growth, they likely played an important role in the recent stock increase. Successful recruitment of these cohorts may have been facilitated by factors such as early hatching and transport to the nursery, which reduces the frequency of predator encounters.

Spatial patterns of phytoplankton community and biomass along the Kuroshio Extension and adjacent waters in late spring

The Kuroshio Extension (KE) is a natural boundary region characterized by complex hydrodynamic processes in the western North Pacific Ocean that fuel spring phytoplankton blooms in high latitudes. This study looked at the spatial patterns and diversity of phytoplankton community and biomass associated with oceanographic profiles across the KE from the north to south in June 2011. Results showed that upper-ocean gradients were strongly correlated with observed spatial variability in phytoplankton communities (> 10 μm). Phytoplankton abundance, species richness, and Chl a biomass were generally higher in the Kuroshio–Oyashio transition region than in the KE axis and the southern KE. No bloom event was recorded in late spring. A total of 81 phytoplankton taxa belonging to 45 genera of four phyla were identified. Dominant taxa in both abundance and occurrence included the diatoms Chaetoceros radicans, C. curvisetus, C. convolutes, C. deblis, C. peruvianus, Pseudonitzchia delicatissima, Coscinodiscus asteromphalus, Thalassionema nitzschioides, T. frauenfeldii, Thalassiothrix longissima, the dinoflagellates Protoperidinium pyrifoprmus, and the Cyanobacterium Trichodesmium thiebautii. Depth-averaged abundance and Chl a were 2076 ± 3,615 cells L−1 and 0.20 ± 0.10 mg m−3, respectively. Subsurface Chl a maxima (SCM) was mainly found at 50 m, while the abundance maximum differed regionally in the water column. A cluster analysis grouped phytoplankton communities into three assemblages related to geographic locations influenced by Oyashio and Kuroshio currents. Nutrient limitation rather than light led to low Chl a and abundance during late spring. Redundancy correspondence and spearman’s correlation further suggest that dissolved inorganic nitrogen was the major factor regulating phytoplankton distribution north of KE, while phosphate limitation was more prominent in the KE axis and the southern KE. The Oyashio and Kuroshio interaction caused evident variability in the hydrodynamic environment including the MLD and Zeu of the region, and changes in chemical profiles such as nutrients, pH, and DO. The observed distributional shift in the spatial pattern of phytoplankton was attributed to upper-ocean environmental gradients within the KE axis and its northern and southern sections.

Otolith oxygen isotope analysis and temperature history in early life stages of the chub mackerel Scomber japonicus in the Kuroshio–Oyashio transition region

To determine the temperature history of early life stages of chub mackerel (Scomber japonicus), otolith stable oxygen isotope ratios (δ18Ootolith) of larval and juvenile fish collected in the Kuroshio–Oyashio transition area during 2004–2015 were analyzed, and experienced temperatures were estimated accordingly. The δ18Ootolith values decreased with otolith growth during the larval stage, whereas those for juveniles were positively correlated with otolith radius and increased with increasing otolith size. Combinations of δ18Ootolith values, average otolith daily increment width, and daily age were representative of the differences between better and poorer recruitment years, the former being characterized by earlier spawning and faster growth (resulting from cooler experienced temperatures), and the latter by slower growth. Analyzed fish were sorted into larval and early juvenile growth stages by cluster analysis; the high-growth cluster exhibited higher δ18Ootolith values than the low-growth cluster. The conversion of δ18Ootolith values to water temperature indicated that the higher growth cluster experienced a water temperature ca. 2.0 °C cooler during the later juvenile stage. Therefore, our results suggest the presence of a growth positive spiral, wherein individuals with high initial growth proactively enter cooler water temperature areas, accessing a highly nutritious diet resulting in further rapid growth.

Variations in the phytoplankton community due to dust additions in eutrophication, LNLC and HNLC oceanic zones

Dust deposition can bring nutrients and trace elements to the upper ocean and affect phytoplankton growth and community structure. We conducted a comparative study using on-board microcosm experiments amended with varying amounts of dust (0.2, 1, and 2 mg L−1) in the East China Sea (eutrophic zone), the subtropical gyre (low-nutrient and low-chlorophyll zone, LNLC), and the Kuroshio-Oyashio transition region (high-nutrient and low-chlorophyll zone, HNLC) of the Northwest Pacific Ocean. The additions of dust supplied a considerable amount of nitrogen (N) and negligible phosphorus (P) relative to the seawater collected for incubation experiments (baseline), contributing to increases in Chlorophyll a with increasing dust additions. Significant linear correlations were observed between the net growth rates of larger cells (i.e., micro-size: >20 μm and nano-size: 2–20 μm) and available N (sum of baseline and added N) at each zone, demonstrating that phytoplankton size structure shifts towards larger cells with the increasing dust additions. In the experiments conducted in LNLC and HNLC zones, micro-sized phytoplankton (primarily consisting of diatoms) benefited most from dust additions. In the experiments conducted in eutrophic zone, however, the primary beneficiary was the nano-sized phytoplankton (primarily consisting of dinoflagellates). When a time lag of one day in relative abundance of diatoms (RAD, the abundance of diatoms divided by the sum of diatoms and dinoflagellates) relative to the N:P ratio was considered, we found the RAD increased substantially with increases in the N:P ratio until the ratio approached the Redfield ratio (N:P = 16:1), and then the RAD decreased gradually as the N:P ratio increased. This was ascribed to the lower sensitivity of dinoflagellates to nutrient shortage, relative to diatoms. Overall, our results suggested that the overwhelming input of N relative to P by dust deposition might cause significant ecological impacts by altering the N:P ratio of varying trophic seawaters.

Spatial and temporal variability in the trans-Pacific migration of Pacific bluefin tuna (Thunnus orientalis) revealed by archival tags

Archival electronic tags were internally implanted in 713 age-0 Pacific bluefin tuna (PBF) caught in their nursery waters off the southern coast of Japan and in the East China Sea over an extended study period (1995–2015) to clarify the spatial and temporal variability of their trans-Pacific migration. Two hundred twenty-five of these tagged tuna were recaptured by fisheries (31.6%), and we successfully retrieved tag data from 14 of 21 individuals recovered in the Eastern pacific. Furthermore, one archival tag recovered in the Western Pacific revealed that the individual had performed a trans-Pacific migration, so in total 21 tagged PBF were shown to have migrated to the Eastern Pacific (2.9% of the total tags released). We successfully downloaded data from 15 of these 21 archival tags, which revealed that some age-1 PBF migrate rapidly (123.9 ± 82.8 km day−1) and directly from waters offshore of Japan to the eastern Pacific (160.0°E to 130.0°W), a journey that takes an average of 2.5 months (ranging from 1.2 to 5.5 months) through relatively cool waters (14.7 ± 2.0 °C). All juvenile PBF began their trans-Pacific migration shortly after exposure to cooler water temperatures (<14 °C), suggesting that sustained residence in lower water temperatures presents a physiological challenge for this age class. Three patterns were identified in the timing of the departure of juvenile PBF from the western Pacific: departing 12–14 months post-hatch (N = 7) in early summer (May-July), departing 17–19 months post-hatch (N = 7) in late autumn (October-December), and departing 21 months post-hatch (N = 1) in late winter (February). The PBF tagged along the southern coast of Japan (SCJ) arrived in the eastern Pacific earlier than those tagged in the East China Sea (ECS), most likely due to the shorter travel distance. Additionally, the PBF that began their trans-Pacific migration in the earlier period remained in an offshore foraging zone (the Kuroshio-Oyashio transition region) for shorter periods (2.8 months on average) and at lower latitudes (35.0°N) during the spring, while the PBF that delayed their migration spent more time (6.7 months on average) in the productive waters between 35.0 and 45.0°N during the spring-autumn months. The variability in the departure timing of the trans-Pacific migration of age-1 PBF may be related to geographic differences between nursery areas in addition to oceanographic conditions and foraging opportunities encountered by the tuna in the offshore waters of Japan during their first year.

Reassessment of Pigmentation Patterns used for Identifying Leptocephali (Gnathophis heterognathos and G. ginanago) Collected from the Kuroshio–Oyashio transition region

Reassessment of Pigmentation Patterns used for Identifying Leptocephali (Gnathophis heterognathos and G. ginanago) Collected from the Kuroshio–Oyashio transition region

Larval and juvenile growth of chub mackerel Scomber japonicus in relation to recruitment in the western North Pacific

To detect indices of recruitment intensity of the Pacific stock of chub mackerel Scomber japonicus, the relationship between the growth rate during early life and recruitment variability and the effects of temperature on annual change of the growth rate were examined. Growth-trajectories in larval and juvenile stages were back-calculated by otolith microstructure analysis using juveniles collected in the Kuroshio–Oyashio transition region of the western North Pacific in May and June from 2002 to 2011. A total of 532 otoliths were analysed. Hatch dates were distributed from mid-March to mid-May and most chub mackerel hatched in April. Mean daily growth rates ± standard deviation (SD) of the April cohort in larval and juvenile stages ranged between 1.03 ±0.10 (2006) and 1.52 ±0.23 mm day−1 (2009). The mean daily growth rate and larval stage duration were significantly related to recruitment which had been estimated by virtual population analysis. The results indicated the growth rate of the April cohort was an important index of year-class strength. There was a significant correlation between sea surface temperature experienced by each fish and the mean daily growth rate during the larval period. Higher water temperature led to faster growth and shorter larval stage duration of the April cohort of the Pacific stock of chub mackerel.

Diving behavior of a false killer whale, Pseudorca crassidens, in the Kuroshio–Oyashio transition region and the Kuroshio front region of the western North Pacific

Diving behavior of a false killer whale, <i>Pseudorca crassidens</i>, in the Kuroshio–Oyashio transition region and the Kuroshio front region of the western North Pacific

Photosynthetic features and primary productivity of phytoplankton in the Oyashio and Kuroshio-Oyashio transition regions of the northwest Pacific

Diel and seasonal changes in the photosynthetic physiology of phytoplankton and primary productivity were investigated during 2005, together with community composition in the Oyashio (OY) and Kuroshio – Oyashio transition (TR) regions of the northwest Pacific. In both regions, diel changes in the maximum photosynthetic rate (P* max) and the light saturation index (Ek) in the photosynthesis – irradiance (P –E) curve were observed, due to diel differences in photo-physiology. In the OY region, the highest values of chlorophyll a concentration, depth-integrated primary production and the maximum quantum yield of carbon fixation in photosynthesis (Fc max) were observed in May when diatom blooms occurred. Furthermore, a higher water-column light utilization efficiency (C) of photosynthesis for phytoplankton was found in the OY region in both May and September. In contrast, in the TR and warm-core ring regions, Fc max was nearly constant, while depth-integrated primary production in May was significantly lower than in the OY region. These results show that the spring phytoplankton assemblages in OY waters had a higher light utilization ability during photosynthesis. Such a high photosynthetic property would contribute to the highest seasonal biological drawdown of surface pCO2 among the world’s oceans (Takahashi, T., Sutherland, S. C., Sweeney, C. et al. (2002) Global sea – air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601– 1622).

Transport and environmental temperature variability of eggs and larvae of the Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus) in the western North Pacific estimated via numerical particle‐tracking experiments

AbstractNumerical particle‐tracking experiments were performed to investigate the transport and variability in environmental temperature experienced by eggs and larvae of Pacific stocks of the Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus) using high‐resolution outputs of the Ocean General Circulation Model for the Earth Simulator (OFES) and the observed distributions of eggs collected from 1978 to 2004. The modeled anchovy individuals tend to be trapped in coastal waters or transported to the Kuroshio–Oyashio transition region. In contrast, a large proportion of the sardines are transported to the Kuroshio Extension. The egg density‐weighted mean environmental temperature until day 30 of the experiment was 20–24°C for the anchovy and 17–20°C for the sardine, which can be explained by spawning areas and seasons, and interannual oceanic variability. Regression analyses revealed that the contribution of environmental temperature to the logarithm of recruitment per spawning (expected to have a negative relationship with the mean mortality coefficient) was significant for both the anchovy and sardine, especially until day 30, which can be regarded as the initial stages of their life cycles. The relationship was quadratic for the anchovy, with an optimal temperature of 21–22°C, and linear for the sardine, with a negative coefficient. Differences in habitat areas and temperature responses between the sardine and anchovy are suggested to be important factors in controlling the dramatic out‐of‐phase fluctuations of these species.

Recruitment Variability of Small Pelagic Fish Populations in the Kuroshio-Oyashio Transition Region of the Western North Pacific

Climatic and ecosystem changes in the transition region between the subarctic Oyashio and the subtropical Kuroshio Extension fronts may be the key to understanding recruitment variability of small pelagic fish populations in the Pacific waters off Japan. The transition region is characterized by complex oceanographic structure with the meandering fronts, eddies and streamers creating large spatial and temporal variability in the environment. Juveniles of small pelagic fishes such as Japanese sardine (Sardinops melanostictus), Pacific saury (Cololabis saira), and Japanese anchovy (Engraulis japonicus) migrate from the subtropical Kuroshio Current area to the subarctic Oyashio Current area across the transition region. A precipitous decline in number of age 0 recruits of Japanese sardine after the end of 1980s appears to be related to the extremely high mortality rate of young-of-the-year fish in the transition region. In contrast, populations of Pacific saury and Japanese anchovy increased after 1988. These synchronous changes in populations seem to be associated with the weakening of the Aleutian Low Pressure in the North Pacific and weakening of the Oyashio Current in winter, and a sea surface temperature (SST) rise in the Kuroshio-Oyashio transition region in spring. The contrasting responses of the fish populations to the SST rise can be explained by different temperature preference in terms of growth rate in larval and early juvenile stages; cool temperatures are preferred by sardine and warm temperatures are preferred by saury and anchovy.

Tempo-spatial patterns of bacterial community composition in the western North Pacific Ocean

In the western North Pacific, where subarctic Oyashio waters encounter subtropical Kuroshio waters, phylotype composition of heterotrophic bacteria was estimated by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified bacterial 16S ribosomal DNA. Total bacterial abundance was also measured by flow cytometry. The study area was divided into four water masses: coastal, Oyashio, Kuroshio, and the Kuroshio–Oyashio transition. Abundances of heterotrophic bacteria in the Oyashio, Kuroshio, and Kuroshio–Oyashio transition regions ranged from 0.2 × 10 5 cells mL − 1 to 1.4 × 10 5 cells mL − 1 and were not significantly different, except in the Oyashio region during September. Bacterial compositions were distinct in each water mass. Furthermore, phylotype distributions differed between surface and subsurface waters in the Kuroshio–Oyashio transition region. Out of 61 DGGE bands obtained, 41 were successfully identified as 31 phylotypes: 22 Gammaproteobacteria, 4 Alphaproteobacteria, 2 unknown bacteria, 2 cyanobacteria, and 1 plastid. Although the Gammaproteobacteria OM60 clade was eurytopic in the study area, Psychrobacter glacincola and the uncultured Gammaproteobacteria SAR92 clade were often observed in the Oyashio region. Overall, our results indicated that Gammaproteobacteria were predominant in the bacterial community, which was influenced by the hydrographic properties of each water mass in the study area.

Differences in cell viabilities of phytoplankton between spring and late summer in the northwest Pacific Ocean

Cell viabilities of phytoplankton in the Oyashio and Kuroshio-Oyashio transition regions of the northwest Pacific Ocean were examined in September 2003 (late summer) and May 2005 (spring) using a membrane permeability test. Specific lysis rates of the phytoplankton during late summer were also assessed by an esterase activity assay. In late summer, cyanobacteria Synechococcus spp. were > 2 × 10 4 cells ml − 1 and numerically dominated the phytoplankton communities. The cell viabilities of Synechococcus spp. and eukaryotic ultraphytoplankton (< 10 μm in size) were 60–79% and 26–41% in surface waters, respectively. The specific lysis rates of the phytoplankton were 0.12–0.67 d − 1 in late summer. By contrast, in spring, eukaryotic cells were predominant in the phytoplankton communities. The cell viabilities of surface eukaryotic ultraphytoplankton in spring were > 70% and significantly higher than those in late summer. During spring, Synechococcus spp. only occurred with < 1 × 10 4 cells ml − 1 in the Kuroshio-Oyashio transition region, and their viabilities were 80%. In the Oyashio region where a spring diatom bloom developed, the viability of fucoxanthin-containing algae (mainly diatoms and prymnesiophytes) was ca. 90%. These results suggested that the cell viability of phytoplankton could vary seasonally with their community structure in the study area. The phytoplankton cell death in late summer was particularly significant for their loss process and could support the microbial food webs by supplying dissolved organic carbon (DOC) derived from the dead cells.

Diet and sexual maturation of the neon flying squid Ommastrephes bartramii during autumn and spring in the Kuroshio–Oyashio transition region

Diet and sexual maturation were examined in the winter–spring cohort of the neon flying squid, Ommastrephes bartramii, during its southward migration through the Kuroshio–Oyashio transition region of the western North Pacific. The main prey items are micronektonic animals and small pelagic fish, which were abundantly distributed throughout the study area. Among the prey species, O. bartramii was dependent on the Japanese anchovy Engraulis japonicus, which also migrate from the northern to southern transition region during the winter, and the micronektonic squid Watasenia scintillans throughout the study period. Other common prey items were Gonatopsis borealis, Diaphus theta, and Ceratoscopelus warmingii from October to November or December, and Tarletonbeania taylori after December. These dietary changes can be explained by the difference in the seasonal north–south migration patterns of the predator and prey species. Male sexual maturation progressed throughout the season, and most individuals were fully mature in January. In contrast, most of the females were immature throughout the study period. The feeding strategy of the squid in relation to their seasonal north to south migration and sexual maturation was discussed.

Latitudinal variation in the recruitment dynamics of small pelagic fishes in the western North Pacific

The subarctic Oyashio Current flows south-westward and the subtropical Kuroshio Current flows north-eastward in the western North Pacific, converging in the waters off northern Japan to form the Kuroshio-Oyashio transition region. Some small pelagic fishes inhabit the subarctic or subtropical waters, and others seasonally migrate north and south across the major ocean fronts. Environmental conditions in the subarctic and transition waters are variable, whereas in the subtropical Kuroshio waters conditions are relatively stable. Latitudinally different environmental conditions may affect vital parameters and recruitment variability of small pelagic fishes inhabiting the various waters. Pacific saury Cololabis saira migrate seasonally from the Kuroshio to Oyashio waters and spawn in the transition waters in autumn and spring and in the Kuroshio waters in winter. During 1990–1999, the coefficients of variation (CVs) of daily growth rates (G) and instantaneous mortality coefficients (M) were large for larvae and juveniles spawned in the northern transition waters, but relatively small for those from the southern Kuroshio waters. The Pacific stock of chub mackerel Scomber japonicus spawns in the Kuroshio waters in spring and early summer and migrates to the subarctic Oyashio waters in summer for feeding, whereas the Tsushima Warm Current stock spawns in the East China Sea in spring and fish remain in the subtropical warm waters throughout their lifetime. The Pacific stock had CVs > 100% for the fish aged 0–5 during 1970–2002. In contrast, the Tsushima Warm Current stock had CVs of 34–40% during 1973–2002. Pacific herring Clupea pallasii, which inhabits subarctic waters, had CVs of 118–178% for the fish aged 3–8 y during 1910–1954. Japanese sardine Sardinops melanostictus, which spawn in the subtropical Kuroshio waters and migrate to the subarctic Oyashio waters in summer for feeding, had CVs > 120% for the fish aged 0–4 during 1976–2003. Contrasting with these subarctic Oyashio associated species, subtropical round herring Etrumeus teres had a CV of 26% over the total catch during 1976–2003. For these small pelagic fishes, latitudinal variation in early life parameters and recruitment was evident at the levels of cohorts ( C. saira), stocks ( S. japonicus), and species (Clupeidae). Large variations were observed in the subarctic Oyashio and the Oyashio-affected transition waters, whereas relatively stable results were recorded for the subtropical Kuroshio and its branch, the Tsushima Warm Current. Because the cohorts of Pacific saury and stocks of chub mackerel are not genetically distinct, it is likely that the large variability in the northern waters was environmentally induced in these species. The differences in CVs among clupeids are probably the result of differences in specific reproductive strategies and the environmental characteristics of the waters inhabited by the different species.

Larval mesopelagic fish assemblages in the Kuroshio–Oyashio transition region of the western North Pacific

We examined larval mesopelagic fish assemblages, their distribution, and seasonal occurrence patterns in the Kuroshio–Oyashio transition region of the western North Pacific where complex hydrographic structures are observed due to the confluence of the Kuroshio Extension and Oyashio current. Larvae of the dominant families Myctophidae, Gonostomatidae, Bathylagidae, Sternoptychidae, and Phosichthyidae were represented by 31 species or types belonging to 24 genera. Based on species composition analysis using the Morishita–Horn similarity index, five assemblages were recognized: Oyashio, Spring Transition, Summer Transition, Kuroshio, and Slope Water assemblages. The distribution patterns of these assemblages corresponded closely with hydrographic structures such as position of the Oyashio and Kuroshio Extension fronts, warm core rings and streamers. Spring Transition (April) and Summer Transition (July–October) assemblages were the most important larval assemblages in the transition region. Larval abundances were low during late autumn and winter. The Spring Transition and Summer Transition assemblages were composed of subtropical, transitional, subarctic, and slope-water species, suggesting the importance of the transition region as nursery grounds of mesopelagic fishes of various origins from subarctic to subtropical waters. Larval fish transport by the Kuroshio, Oyashio, and Tsugaru Warm currents into the transition region is discussed.

Environmental determinants of growth rates for larval Japanese anchovy Engraulis japonicus in different waters

AbstractDo disparate mechanisms determine growth rates of fish larvae in the different regions? The relationship between growth rates and environmental factors (sea temperature and food availability) was examined for larval Japanese anchovy Engraulis japonicus in geographically and environmentally different waters, through sagittal otolith microstructure analysis. Recent 3‐day mean growth rates directly before capture were positively related with sea‐surface temperature (SST) but not with food availability (plankton density) for the larvae in the Kuroshio Extension and Kuroshio–Oyashio transition regions of the western North Pacific. On the contrary, variations in recent growth rates were attributed to food availability (plankton density) as well as SST for the larvae in the East China Sea. In the shirasu fishing ground in Sagami Bay, larval growth rates were variable under the influences of both SST and food availability (feeding incidence). On the surface, the growth–environment relationships seemed to differ among regions. However, a definite general pattern of the dome‐shaped relationship between recent growth rates and SST was observed when all the regions were combined. Growth rates were similar even among clearly different regions if at the same SST. Overall, growth rates roughly increased with SST until they reached the maximum at SST of 21–22°C (i.e. optimal growth temperature), and declined when SST went over 21–22°C. On the contrary, no clear relationship was observed between growth rate and plankton density or between SST and plankton density. Therefore, the apparent among‐region differences would be firstly caused by the differences in regional SST range. The systematic mechanism of growth determination for widespread pelagic fish species larvae would be run by primarily sea temperature and secondarily food availability, at the species level.

Effects of temperature and food availability on growth rate during late larval stage of Japanese anchovy (Engraulis japonicus) in the Kuroshio–Oyashio transition region

AbstractDuring periods when the population size of Japanese anchovy Engraulis japonicus is large, the abundance of 1‐yr olds has been considered to be dependent on the growth and survival processes in the late larval and early juvenile stages in the Kuroshio–Oyashio transition region off northern Japan. Recent growth rates for 10 days before capture of larval and early juvenile E. japonicus were estimated and examined in relation to the surface water temperature and the available copepod density in 1997, 1998 and 1999. Late larval and early juvenile E. japonicus were distributed in the waters with temperature from 15 to 19°C and available prey density from 10 to 1000 mg dry weight (DW) m−2 in the transition region. The late larval growth rates were found to be regulated more strongly by water temperature than by copepod density in the waters &lt;16°C, and more strongly by copepod density than water temperature in the waters &lt;100 mg DW m−2 in the Kuroshio–Oyashio transition region. The recent growth rates decreased from the western waters to the eastern waters in the survey area 140–170°E in 1998, correlating with decreases of food availability to 50–100 mg DW m−2. While in 1999, the recent growth rates were faster in the waters east of 150°E, resulting from eastward expansion of warm water ranges and high available prey density 100–400 mg DW m−2. The key environmental factors regulating late larval growth rate of E. japonicus in the transition region seem to be spatially different between years.

Contributions of the VENFISH program: meso‐zooplankton, Pacific saury (Cololabis saira) and walleye pollock (Theragra chalcogramma) in the northwestern Pacific

AbstractThe comprehensive study of the variation of the oceanic environment and fish populations in the northwestern Pacific (VENFISH) was an interdisciplinary scientific program to enhance our understanding of relationships between the ocean environment and the pelagic fishes, Pacific saury and walleye pollock. As prey of saury and walleye pollock, key species of zooplankton were selected and their life history and production were elucidated. Retrospective analyses were also conducted to investigate the mechanisms of production variation in some zooplankton species. Life history and ecology of Pacific saury and walleye pollock were also investigated and substantial improvements to our understanding of them were achieved. In addition, improved information from VENFISH were used to develop several models to investigate saury growth, and population dynamics, and predatory pressure on zooplankton from walleye pollock. Observational and model results indicate the importance of the Kuroshio and the Kuroshio–Oyashio transition region to saury recruitment and of Oyashio transport to walleye pollock recruitment. Moreover, models developed under VENFISH will become the basis of fisheries management models for saury, walleye pollock, and other fishes in the future.